For those that have something to sell, based on personal experience, I've really liked using
https://macmeanoffer.com. They will buy phones, macs, apple watches, etc. When I needed to sell, Apple's offering was so much lower than what I was seeing on other sites. But it felt super scary to go the eBay route or how to even choose from the dozen sites that buy these used products. But I decided to go for it, and I used them twice in 2019 for an iPhone, and for a MacBook. It went super smoothly and I highly recommend them based on my experiences.
On the site, you fill out the info of your device like normal (like whether you have the original box, original cables, etc.) and it will give you an estimate. But it'll say to wait until the next business day for the final estimate to be created by a person (based on what you're including and any special notes), along with a prepaid shipping label that I believe they want you to ship within 3 days of receiving the quote. Otherwise, if it expires, you can request a new quote. I did this once when I couldn't get my Macbook wiped in time, so I did another quote a week later and got quoted the same amount, no problem.
I'll say that for my phone, since I did not have the original box, they took off about $10 from the original quote to when their employee sent me the final quote with a shipping label. It was still better than anyone else, so I sent it off. For my mac mini, since I did have the original box, the estimate was spot on. After shipping, a few days after they received the package I got an email saying everything was as described and they were sending the payment. A few days later I received the check in the mail. I believe they offer PayPal as an alternative, but they make us eat the 2.9%ish fee, so I was hesitant with requesting a check, but both times I chose check it was fine.
They seem to especially offer a bit more for slightly older hardware, as 'last years model' seems to be a little closer, but still higher.